﻿342 Mr. J. C. Douglas on Shadow Optometers, . 



out the interposition of a lens; but hypermetropia cannot be so 

 measured, there being no positive distance at which vision is 

 perfect. If a graduated series of lenses be not at hand, it must 

 be first overcorrected by a lens, and then the distance of the 

 focus of incident rays adjusted until the shadow is no longer seen. 



If two or more luminous points be used, they are removed 

 until the shadows are erect and move in the same direction as 

 the object (a convex lens being employed in hypermetropia). The 

 object being held near the eye, as many shadows are seen as 

 lights employed ; the object is now removed until one shadow 

 only is seen ; the distance at which this occurs, and the effect 

 of any lens introduced being known, the distance required may 

 be at once arrived at. 



On comparing the methods last referred to with Father Shei- 

 ner's method, I found in my own case a difference of one inch in 

 favour of the proposed method in determining the far point — 

 Sheiner's method giving rather more than seven, and the mul- 

 tiple-shadow method proposed more than eight inches. I found, 

 in Sheiner's experiment, that practice enabled me to render the 

 object confused after it had apparently reached the far point, and 

 that it became again distinct after further removal ; but this dis- 

 tance was that referred to above, and was not the far point ; for 

 on regarding an approaching point of light, I found the distance 

 thus obtained agreed with the shadow indication, and exceeded 

 therefore the distance given by Sheiner's experiment. In some 

 hundreds of experiments Sheiner's method invariably gave a 

 shorter distance; the interposition of a concave lens removed 

 the far point obtained by the shadow method to twelve inches ; 

 but that obtained by Sheiner's method under the same circum- 

 stances was about an inch less than this distance. 



The method proposed appears to have the following advan- 

 tages over Father Sheiner's experiment : it appears more accu- 

 rate; vision is natural, not being modified by the introduction of 

 a diaphragm ; it is at once seen what kind of correction is required ; 

 and the far point may be more readily found. Most persons ac- 

 commodate for their far point only when actually viewing a dis- 

 tant object ; and the conditions of Sheiner's experiment are there- 

 fore unfavourable to the determination of the far point ; but no 

 unfavourable conditions are introduced in the methods proposed, 

 a distant object being viewed. It also appears superior to the 

 use of test-types, being more accurate, showing more certainly 

 (to unscientific persons at least) the existence of confusion and 

 the kind of correction required. In the last two respects it is 

 superior to the method recommended by Bonders, viz. view- 

 ing a point of light through lenses, the passing of the object 

 showing at once if perfect adjustment has been arrived at, and, 



